U.S. patent number 6,768,756 [Application Number 09/804,618] was granted by the patent office on 2004-07-27 for mems membrane with integral mirror/lens.
This patent grant is currently assigned to Axsun Technologies, Inc.. Invention is credited to Dale C. Flanders, Steven F. Nagle, Margaret B. Stern.
United States Patent |
6,768,756 |
Flanders , et al. |
July 27, 2004 |
MEMS membrane with integral mirror/lens
Abstract
An optical membrane device and method for making such a device
are described. This membrane is notable in that it comprises an
optically curved surface. In some embodiments, this curved optical
surface is optically concave and coated, for example, with a highly
reflecting (HR) coating to create a curved mirror. In other
embodiments, the optical surface is optically convex and coated
with, preferably, an antireflective (AR) coating to function as a
refractive or diffractive lens.
Inventors: |
Flanders; Dale C. (Lexington,
MA), Nagle; Steven F. (Cambridge, MA), Stern; Margaret
B. (Sudbury, MA) |
Assignee: |
Axsun Technologies, Inc.
(Billerica, MA)
|
Family
ID: |
25189415 |
Appl.
No.: |
09/804,618 |
Filed: |
March 12, 2001 |
Current U.S.
Class: |
372/43.01;
372/101; 372/29.022; 372/75; 372/99 |
Current CPC
Class: |
G02B
5/1828 (20130101); G02B 26/001 (20130101); G02B
26/02 (20130101); G02B 26/0833 (20130101); G02B
26/0875 (20130101) |
Current International
Class: |
G02B
26/08 (20060101); G02B 26/00 (20060101); G02B
26/02 (20060101); G02B 5/18 (20060101); H01S
003/13 (); H01S 003/08 () |
Field of
Search: |
;372/43,75,29.022,99,101,107 ;359/579 ;365/215 |
References Cited
[Referenced By]
U.S. Patent Documents
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5245622 |
September 1993 |
Jewell et al. |
5291502 |
March 1994 |
Pezeshki et al. |
6201629 |
March 2001 |
McClelland et al. |
6271052 |
August 2001 |
Miller et al. |
|
Foreign Patent Documents
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198 51 967 |
|
May 2000 |
|
DE |
|
0 420 468 |
|
Apr 1991 |
|
EP |
|
0 523 861 |
|
Jan 1993 |
|
EP |
|
WO 99/34484 |
|
Jul 1999 |
|
WO |
|
01/09995 |
|
Feb 2001 |
|
WO |
|
Other References
Hisanaga, Michio, et al., "Fabrication of 3-Dimensionally Shaped Si
Diaphragm Dynamic Focusing Mirror," Proceedings of the Workshop on
Micro Electro Mechanical Systems (MEMS) IEEE, vol. 6, pp. 30-35
(Feb. 7-10, 1993). .
Fletcher, Daniel A.; Crozier, Kenneth B., Kino, Gordon S.; Quate,
Calvin F.; Goodson, Kenneth E., "Micromachined Scanning Refractive
Lens," Solid-State Sensor and Actuator Workshop, Hilton Head
Island, South Carolina, Jun. 4-8, 2000..
|
Primary Examiner: Jackson; Jerome
Assistant Examiner: Nguyen; Joseph
Attorney, Agent or Firm: Houston; J. Grant
Claims
What is claimed is:
1. An optical membrane device comprising a support; a device layer
in which a deflectable membrane structure is formed; a sacrificial
layer separating the support from the device layer, the sacrificial
layer being selectively removed to release the membrane structure;
and an optically curved surface on the deflectable membrane and on
an optical axis of the optical membrane device.
2. An optical membrane device as claimed in claim 1, wherein the
optical surface is formed in an optical element layer that is
deposited on the device layer.
3. An optical membrane device as claimed in claim 1, wherein the
optical surface is etched into the device layer.
4. An optical membrane device as claimed in claim 1, wherein the
optical surface is a concave surface that is formed into the device
layer.
5. An optical membrane device as claimed in claim 1, wherein the
optical surface is a convex surface that is etched into the device
layer.
6. An optical membrane device as claimed in claim 5, wherein the
sacrificial layer defines an electrical cavity across which
electrical fields are established to deflect the membrane structure
in a direction of the support.
7. An optical membrane device as claimed in claim 6, wherein the
membrane structure comprises: a center body portion; an outer
portion, which is at least partially supported by the sacrificial
layer; and tethers that extend between the center body portion and
the outer portion.
8. An optical membrane device as claimed in claim 1, wherein the
sacrificial layer defines an electrical cavity across which
electrical fields are established to deflect the membrane structure
in a direction of the support.
9. An optical membrane device as claimed in claim 1, wherein the
membrane structure comprises: a center body portion; an outer
portion, which is at least partially supported by the sacrificial
layer; and tethers that extend between the center body portion and
the outer portion.
10. An optical membrane device as claimed in claim 1, further
comprising an optical coating deposited over the optical
surface.
11. An optical membrane device as claimed in claim 10, wherein the
optical coating is multilayer dielectric mirror.
12. An optical membrane device as claimed in claim 10, wherein the
optical coating is an antireflective coating.
13. An optical membrane device as claimed in claim 1, wherein the
optically curved surface of the deflectable membrane is centered on
the optical axis of the optical membrane device.
14. An optical membrane device as claimed in claim 1, wherein the
optically curved surface of the deflectable membrane is centered on
the deflectable membrane.
Description
BACKGROUND OF THE INVENTION
Micro-optical electromechanical system (MEOMS) membranes are used
in a spectrum of optical applications. For example, they can be
coated to be reflective and then paired with a stationary mirror to
form a tunable Fabry-Perot (FP) cavity/filter. They can also be
used as stand-alone reflective components to define the end of a
laser cavity, for example.
Typically, membrane deflection is achieved by applying a voltage
between the membrane and a fixed electrode. Electrostatic
attraction moves the membrane in the direction of the fixed
electrode as a function of the applied voltage. This results in
changes in the reflector separation of the FP filter or cavity
length in the case of a laser.
In optical systems, these membranes have advantages over
cantilevered structures, for example. Membranes better maintain
parallelism through the range of their deflection and tend to be
more mechanically robust and have fewer relevant vibration
modes.
In the past, the commercial MEOMS membranes have been produced by
depositing a dielectric mirror structure over a sacrificial layer,
which has been deposited on a support structure. This sacrificial
layer is subsequently etched away to produce a suspended membrane
structure in a release process. If a curved membrane structure is
desirable, a compressive stress is cultivated in the silicon
compound to induce a bow.
SUMMARY OF THE INVENTION
In number of applications, it would be desirable to fabricate
membranes with predetermined optically curved surfaces that work in
transmission or reflection, such as 1) refractive lens structures,
including lenses with continuous curvatures or Fresnel profiles; 2)
diffractive lens or mirror structures; or 3) mirror structures
having continuous curvatures or Fresnel profiles.
The present invention concerns an optical membrane device and
method for making such a device. This membrane is notable in that
it comprises an optically curved surface. In some embodiments, this
surface is optically concave and coated, for example, with a highly
reflecting (HR) coating to create a curved mirror. In other
embodiments, the optical surface is optically convex and coated
with, preferably, an antireflective (AR) coating to function as a
collimating or focusing lens.
In general, according to one aspect, the invention features an
optical membrane device that comprises a support and a device
layer, wherein a deflectable membrane structure is formed in the
device layer. A sacrificial layer separates the support from the
device layer. This sacrificial layer has been selectively removed
to release the membrane structure. According to the invention, an
optically curved surface has been formed on this deflectable
membrane.
In one example, the curved optical surface is formed in an optical
element layer that is deposited on the device layer. Alternatively,
the curved optical surface is etched into the device layer.
In one implementation, the curved optical surface is an optically
concave surface that has been etched into the device layer. This
optically concave surface is formed as either a continuously
curved, a Fresnel, or diffractive surface. In another
implementation, the optical surface is an optically convex shape
that is formed in a layer that has been deposited on the devices
layer or that has been etched into the device layer. This optically
convex shape is formed either as a continuous curved surface, a
Fresnel surface or using diffractive features.
In the present implementation, the sacrificial layer defines an
electrical cavity across which electrical fields are established to
deflect the membrane structure in the direction of the support or
stationary electrode. In one example, this membrane structure
comprises a center body portion, an outer portion, and tethers that
extend between the center body portion and the outer portion.
In one application, an optical coating is applied to the optically
curved surface. For example, a concave mirror structure is formed
by applying a multi-layer dielectric mirror coating to the curved
optical surface. In another example, an anti-reflective coating is
applied to a convex optical surface to thereby form a refractive or
diffractive lens element.
In general, according to another aspect, the invention also
features a process for fabricating an optical membrane structure.
This process comprises providing a support and forming a
sacrificial layer on the support. A device layer is then further
formed on this sacrificial layer. A membrane structure is patterned
into the device layer and the membrane structure released by the
selective removal of the sacrificial layer. Finally, according to
the invention, an optically curved surface is formed on the
membrane structure of the device layer.
In one embodiment, the process for fabricating the curved optical
surface comprises depositing a photo-resist layer and then
reflowing that photo-resist layer to create a curved surface. This
curved surface is then transferred into the device layer by etching
the photo-resist and the device layer.
To form a convex optical surface, a columnar photo-resist layer is
reflowed to form a convex surface. In contrast, to create a concave
surface, a columnar blind hole is etched into a photo-resist layer
or the device layer. This columnar blind hole is then over-coated
with another photo-resist layer and then the resulting concave
surface transferred into the device layer by etching.
In general, according to still another aspect, the invention
features a process for fabricating concave mirror structures. This
process comprises depositing a photo-resist layer over a well or
blind hole in a substrate. In one example, this substrate is
limited to simply a device layer. In another example, this
substrate comprises a multi-layer structure, such as a device
layer, which has been coated with another photo-resist layer. A
resulting curved surface is then transferred into the substrate by
etching the photo-resist and the substrate. This curved surface is
then coated with a dielectric mirror coating to thereby yield a
concave mirror structure.
The above and other features of the invention including various
novel details of construction and combinations of parts, and other
advantages, will now be more particularly described with reference
to the accompanying drawings and pointed out in the claims. It will
be understood that the particular method and device embodying the
invention are shown by way of illustration and not as a limitation
of the invention. The principles and features of this invention may
be employed in various and numerous embodiments without departing
from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, reference characters refer to the
same parts throughout the different views. The drawings are not
necessarily to scale; emphasis has instead been placed upon
illustrating the principles of the invention. Of the drawings:
FIG. 1 is a perspective, exploded view of a tunable filter
comprising an optical membrane device with an optically curved
mirror structure, according to the present invention;
FIG. 2 is a perspective view of the inventive optical membrane
device including an optically convex or concave surface and showing
the backside optical port, in phantom;
FIG. 3 is an elevation view of the distal side of the inventive
optical membrane device showing the optical port with the lens or
mirror structure, in phantom;
FIGS. 4A through 4D are schematic cross-sectional views
illustrating a process for fabricating a membrane device according
to the present invention;
FIGS. 5A through 5E are detailed schematic cross-sectional views
illustrating the fabrication for the membrane optical element
according to a first embodiment;
FIGS. 6A through 6C are detailed schematic cross-sectional views
illustrating the fabrication for the membrane optical element
according to a second embodiment; and
FIGS. 7A through 7B is detailed schematic cross-sectional views
illustrating the fabrication for the membrane optical element
according to a third embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a Fabry-Perot tunable filter 100 comprising an optical
membrane device 110, which has been constructed according to the
principles of the present invention.
Generally, in the FP filter 100, a spacer device 114 separates the
mirror device 112 from the membrane device 110 to thereby define a
Fabry-Perot (FP) cavity.
The optical membrane device 110 comprises handle material 210 that
functions as a support. Preferably, the handle material is wafer
material such as from a silicon handle wafer, which has been
subsequently singulated into the illustrated device.
An optical membrane or device layer 212 is added to the handle
wafer material 210. The membrane structure 214 is formed in this
optical membrane layer 212. In the current implementation, the
membrane layer 212 is silicon. An insulating layer 216 separates
the optical membrane layer 212 from the handle wafer material
210.
During manufacture, the insulating layer 216 functions as a
sacrificial/release layer, which is partially removed to release
the membrane structure 214 from the handle wafer material 210.
Currently, the membrane layer is manufactured from a silicon wafer
that has been bonded to the insulating layer under elevated heat
and pressure.
In the current embodiment, the membrane structure 214 comprises a
body portion 218. The optical axis 10 of the device 100 passes
concentrically through this body portion 218 and orthogonal to a
plane defined by the membrane layer 212. A diameter of this body
portion 218 is preferably 300 to 600 micrometers; currently it is
about 500 micrometers.
Tethers 220 extend radially from the body portion 218 to an outer
portion 222, which comprises the ring where the tethers 220
terminate. In the current embodiment, a spiral tether pattern is
used.
According to the invention, an optically curved surface 250 is
disposed on the membrane structure 214. This optically curved
surface 250 forms an optical element. In the illustrated
implementation of the membrane device 110, the surface of the
optical element is optically concave to thereby form a curved
mirror/flat mirror FP filter cavity in conjunction with the mirror
device 112.
An optical coating dot 230 is typically deposited on the body
portion 218 of the membrane structure 214, specifically covering
the optically curved surface 250 of the optical element. In the
implementation as a Fabry-Perot filter or other reflecting
membrane, the optical dot 230 is preferably a highly reflecting
(HR) dielectric mirror stack, comprising 6 or more layers of
alternating high and low index material. This yields a highly
reflecting, but low absorption, structure that is desirable in, for
example, the manufacture of high finesse Fabry-Perot filters.
In the illustrated embodiment, artifacts of the manufacture of the
membrane structure 214 are etchant holes 232. These holes allow an
etchant to pass through the body portion 218 of the membrane
structure 214 to assist in the removal of the insulating layer 216
during the release process.
In the illustrated embodiment, metal pads 234 are deposited on the
proximal side of the membrane device 210. These are used to solder
bond, for example, the spacing structure 214 onto the proximal face
of the membrane device 210, which could be avoided if the spacing
structure 214 is formed to be integral with the membrane device 110
or mirror device 112. Bond pads 234 are also useful when installing
the filter 100 on a micro-optical bench, for example. Also provided
are a membrane layer wire bond pad 334 and a handle wafer wire bond
pad 336. The membrane layer bond pad is a wire bonding location for
electrical control of the membrane layer. The handle wafer bond pad
336 is a wire bond pad for electrical access to the handle wafer
material.
FIG. 2 illustrates the membrane device 110 with the optically
curved surface 250 of the optical element in an unpaired
configuration, i.e., without mirror device 110. In this
configuration, an optically concave surface 250, providing a
concave optical element, is used as, for example, a reflector in a
laser cavity. In such an application, an optical port 240 (shown in
phantom) is provided, extending from a distal side of the handle
wafer material 210 to the membrane structure 214 in cases where the
reflector is used as an output reflector or to provide for
monitoring. If the reflector is used as a back reflector, then the
port 240 is not necessary in some cases.
Further, whether or not this optical port 214 is required also
depends upon the transmissivity of the handle wafer material 210 at
the optical wavelengths over which the membrane structure 110 must
operate. Typically, with no port, the handle wafer material along
the optical axis must be AR coated if transmission through the
backside is required for functionality.
In another configuration, the optical surface 250 of the optical
element is an optically convex surface and AR coated to function as
a collimating or focusing lens optical element, a position of which
is modulated along the optical axis 10 to thereby control the
location of a focal point or beam waist, in one application.
According to the invention, the optically convex and optically
concave surface 250 is formed either as a surface with a continuous
curvature, a stepped curvature of a Fresnel structure, or with
refractive structures.
FIG. 3 shows the optical port 240 formed through the distal side of
the handle wafer material 210 in the optical membrane device 110,
if necessary or desirable. Specifically, the optical port 240 has
generally inward sloping sidewalls 244 that end in the port opening
246. As a result, looking through the distal side of the handle
wafer material, the body portion 218 of the membrane structure is
observed. The port is preferably concentric with the optical
coating 230 and the optical surface 250.
FIGS. 4A through 4D illustrate a process for fabricating a membrane
device according to the present invention.
Referring to FIG. 4A, the process begins with a support or handle
wafer 210, which in one embodiment is a standard n-type doped
silicon wafer. The handle wafer 210 is 75 mm to 150 mm in diameter,
for example.
The wafer 210 is oxidized to form the sacrificial insulating layer
216. The sacrificial insulating layer 216 defines the electrostatic
cavity in the illustrated embodiment. Presently, the insulating
layer is 216 is between 3.0 and 6.0 .mu.m.
The membrane or device layer 212 is then deposited or otherwise
installed on the sacrificial insulating layer 216. Preferably, the
membrane layer 212 is 6 to 10 .mu.m in thickness. Such thickness
range provides adequate structural integrity while not making the
structure overly rigid or brittle. This layer can be formed from a
range of materials such as silicon wafer material or
polysilicon.
The membrane layer 212 can be annealed and polished back to the
desired membrane thickness, if necessary. A thin oxide layer 416 is
preferably then grown on the membrane layer 212 to function as an
etch protection layer.
As shown in FIG. 4B, the optical port 240 is patterned and etched
into the handle or support wafer 210 in a backside etch process,
preferably using a combination of isotropic and anisotropic
etching. The sacrificial insulating layer 216 is used as an etch
stop.
Alternatively, the optical port etch step can be omitted, as
silicon is partially transparent at infrared wavelengths. In such
implementation, an anti-reflective (AR) coating is applied to the
outer surface of handle wafer 210 and other air-silicon interfaces
to minimize reflection from the interfaces.
FIG. 4C illustrates formation of the optical surface 250, which is
concave in the illustrated example. The highly reflective (HR) spot
230 is then deposited on the optical surface in the case of a
reflector. AR coatings are typically used with lens. Specifically,
the HR spot 230 is formed by depositing and etching-back using a
patterned photo-resist layer or deposition through a shadow mask.
The HR coating is preferably a multi-layer coating of 4 more
layers, preferably 8 or more, with a 16 dielectric layer mirror
being used in the current embodiment. The preferred method of
etching the dielectric coatings 230 is to use a dry etch process,
such as reactive ion etching or reactive ion milling.
Also shown is the formation membrane structure including the
tethers 220, membrane body 218, and outer portion 222 in the
membrane layer 212. Specifically, a photoresist layer is deposited
and patterned with the membrane structure pattern. It also
functions to protect the HR spot 230, in one embodiment. The
release process is performed in which an etchant is used to remove
the insulation layer 212 from underneath the membrane
structure.
Finally, as shown in FIG. 4D, an anti-reflection (AR) coating 420
is deposited through the optical port 240 onto the exterior surface
of the membrane. Further, metal pads 234 are added.
FIG. 5A is a partial cross-sectional view showing the device layer
212 and the sacrificial layer 216 in the region of the device layer
where the optical surface is to be formed.
FIG. 5B shows a first step of the process in which a patterning
layer, such as a photo-resist layer 510 is deposited on the device
layer 212.
As illustrated in FIG. 5C, the photo-resist layer 510 is patterned
to have a columnar blind hole 512. In the illustrated example, this
columnar hole extends to the device layer 212.
A second photo-resist layer 514 is then deposited and spun-over the
first photo-resist 510 and specifically, the blind hole 512. The
surface characteristics of this concave region 516 can optionally
be improved by reflowing the resist under elevated temperatures. In
the preferred embodiment, resist layers 510 and 512 are selected to
have similar or the same etch rates as the device layer 212.
As a result, as illustrated in FIG. 5D, when the device is exposed
to a nominally anisotropic, non-selective etch, the curved surface
516 is transferred into the device layer 512 to thereby form the
optically curved surface 250. In other embodiments, partially
selective etches are used to modify the curved surface during the
transfer.
Subsequently, as illustrated in 5E, the optical surface 250 is then
coated with an optical coating 230. Depending on the
implementation, this optical coating 230 is either an AR or HR
coating.
FIGS. 6A-6C illustrate other techniques for fabricating the optical
surface. In this example, a blind hole 610 is etched into the
device layer 212.
This blind hold 610 can be directly converted into the curved
optical surface 250 that is illustrated in FIG. 6C by a mass
transport process. Such mass transport processes, however, assume
that the device can withstand the required high processing
temperatures, which may not be true in all instances.
As a result, the more typical implementation is illustrated in FIG.
6B. Specifically, a photoresist layer 510 is deposited over the
device layer 512 similar to that step as illustrated in FIG. 5C.
The curved surface 516 is created using the conformal deposition or
facilitated with resist reflow. The resist layer 510 and the device
layer 512 are then etched to transfer the curved optical surface
250 into the remainder of the device layer 212.
FIGS. 7A and 7B illustrate a related process for fabricating a
convex surface on the device layer 212. Specifically, a columnar
structure 710 is formed on the device layer 212 by etching the
surrounding regions of the device layer 212. Then, a photoresist
layer 712 is deposited over the device layer 212 and reflowed, if
necessary.
Then, as illustrated in FIG. 7B, the photoresist 712 and device
layer 212 are etched to transfer the curved surface 714 of the
photo-resist 712 into the device layer 212.
While this invention has been particularly shown and described with
references to preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the scope of the
invention encompassed by the appended claims. For example, in still
other implementations, the photo resist is patterned using
gray-scaling techniques.
* * * * *